In the 1970s, the term glycemic index (GI) was first introduced to the nutrition community as a way to classify carbohydrates (CHO) based on their physiologic effect on blood glucose levels. Prior to this, CHO were (and still are), categorized based on chemical structure. Most notably, when stratified by chemical composition, CHO can be thought of as sugars, starches, and fibers. In clinical settings, the GI has proven to be a more useful nutritional concept than the chemical classification of CHO, as it provides relevant information for the prevention and treatment of chronic disease. Since then, the use of the GI to promote weight loss, improve chronic health conditions, and fuel athletic performance has been studied by numerous researchers in a variety of settings (3,5,17,21,25).
Other ways to categorize CHO have been promoted by the food industry; sometimes referring to their products as low in “sugars” or “impact carbs.” These terms can be defined as CHO that are high on the GI, and have a great impact on blood glucose levels by causing a large release of insulin. Other times CHO are reported as “simple” versus “complex.” Simple CHO are mono-, di-, or oligo-saccharides, often times referred to as the “bad carbs.” These CHO are lumped into a broad category reported to cause large, rapid changes in blood glucose levels, followed by a greater fall in blood glucose. The end result is a temporary over production of insulin that can result in hypoglycemia. Complex CHO, or the “good carbs,” are polysaccharides or starches and are usually touted as containing significant amounts of other nutrients, including dietary fiber, making them more nutrient dense than simple CHO. Since the digestion and absorption of complex CHO foods are slower, they produce more stable or sustained blood glucose and insulin responses. Theoretically, these responses would be more desirable for athletes during sustained competition.
Unfortunately, classifying CHO as simple or complex is flawed, confusing, and inaccurate; especially when these terms are used as surrogates for GI. To begin, most foods are a combination of simple and complex CHO, and so this strict dichotomy only works for a select few food items. Furthermore, to label simple CHO as unhealthy and complex CHO as healthy is simply not accurate. Contrast the simple sugars found in fruits to the complex CHO found in foods such as pizza, fries, and potato chips. Despite their simple sugar content, fruits are considered healthy foods as they provide a good source of select vitamins, minerals, and fiber. Conversely, pizza, fries, and potato chips are less nutritious and higher in fat than other “simple sugar” foods (such as fruit), despite their “complex CHO” label. Using this classification system Americans have been falsely educated to correlate simple sugars with negative health outcomes, and complex CHO with positive health outcomes. Clearly, a better system is needed to classify and discuss CHO.
We propose classifying CHO into “refined” (processed foods) and “unrefined” (natural foods) categories. Refining is a process that can apply to simple or complex CHO. Quite literally, it is the act of removing fiber, nutrients, usually water, and other items contained within the food in its natural state. The act of refining foods typically concentrates sugars and results in food items with a heightened GI. Unrefined CHO are usually healthy, unadulterated, foods such as fruits, vegetables, and whole grains, which are lower in the GI, and result in a more flattened blood glucose response (Table 1).
GLYCEMIC INDEX VERSUS GLYCEMIC LOAD
By definition, GI is a ranking of foods based on their actual postprandial blood glucose response compared to a reference food, either glucose or white bread. GI is calculated by measuring the incremental area under the blood glucose curve following ingestion of a test food providing 50 g of CHO, compared with the area under the curve following an equal CHO intake from the reference food, with all tests being conducted after an overnight fast.
Simply put, GI reflects the rate of digestion and absorption of a CHO-rich food. Numerous factors have been identified that influenced the digestion and absorption rate of CHO rich foods, including degree of food processing and cooking, the presence of fructose or lactose, the ratio of amylopectin and amylase in starch, starch-protein or starch-fat interactions, and the presence of anti-nutrients such as phytates and lectins (5,26). In 1981, Jenkins et al. produced table showing GI response to 62 commonly eaten foods (15), and based on this research, the concept of stratifying CHO by their GI ranking was born; with a GI score of 55 or less constituting a low GI food, 56-69 a medium GI food, and 70 or higher a high GI food.
It has been argued that the GI lacks practical utility as the amount of CHO ingested also affects circulating blood sugar. Thus, to capture the entire glucose-raising potential of dietary CHO, the concept of glycemic load (GL) was introduced to simultaneously incorporate both the quality and the quantity of CHO consumed. In essence, GL is a function of the GI of a certain food multiplied by the number of grams of CHO of that food in a single serving. We believe the GL represents a more relative and accurate effect a food has on blood glucose.
Proponents of the GI/GL based diets argue that manipulating dietary prescription to include low GI/GL foods decreases the occurrence of hyperglycemia and hyperinsulinemia, both of which can lead to insulin resistance and further chronic disease. Applications from this area of research were then extended to the athlete; with emphasis placed on exploiting the principles of GI and GL to maximize endogenous CHO stores, thereby minimizing the potential ergolytic effects of CHO depletion (11). In the sections that follow, current research in this area will be summarized, with emphasis placed on the role of GL in guiding athlete's dietary food choices pre-, during, and post-exercise.
Previous literature has shown that ingestion of a CHO-rich meal (∼200-300 g) within four hours of exercise improves endurance or performance during prolonged moderate intensity exercise (7,19,20,27) presumably due to increases muscle glycogen (10). However, recent literature suggests that when CHO is ingested during exercise in amounts presently recommended by sports nutrition guidelines, pre-exercise CHO intake has little effect on metabolism or on subsequent performance during prolonged cycling (4).
It has also been speculated that the negative outcomes of CHO ingestion 30 to 60 minutes prior to exercise (i.e., increase muscle glycogen utilization and decreased time to fatigue) may be avoided by consuming a low GI food (24). Moreover, low GI foods may be favored one hour prior to exercise because the slow digestion of these foods results in the availability of fuels near the end of exercise (25).
Athletes undertaking prolonged exercise are advised to consume CHO during the event to enhance performance, most often as diluted liquids. In general, ∼1 g CHO·min−1 is advised, since this appears to be the maximal rate of oxidation of ingested CHO (5). Although it would make sense that CHO consumed during exercise should be easily digested and absorbed to provide a rapid supply of energy (8), the effect of the GI and GL of CHO rich foods/drinks during exercise has not yet been systematically studied. That being said, many athletes choose CHO sources, including specially manufactured sports drinks and bars, which would be classified as moderate to high in GI or GL (13). Moreover, low GI/GL foods tend to cause more stomach distress and are avoided during exercise. Studies need to be undertaken to determine if low GI/GL CHO are advantageous over high GI/GL CHO before and during exercise.
Previous literature has shown that the highest rate of muscle glycogen storage occurs during the first hour after exercise (14,16). Moreover, the most important dietary factor in glycogen recovery during this immediate post-exercise period is the amount of CHO that is consumed, with an intake of ∼7-10 g · kg−1 body mass providing maximal daily glycogen storage (9). These findings inherently lend themselves to high GL foods, regardless of GI rank. Additionally, high GI CHO consumed together with protein have been shown to enhance exercise-induced muscle formation when given immediately post exercise (23). In a recent study by Levenhagen et al., ten subjects were used in a crossover design to examine the protein dynamics of early vs. late macronutrient post-exercise supplementation (18). The same oral supplement (10 g protein, 8 g CHO, 3 g fat) was administered for both trials, either immediately or three hours post-exercise. Markers of protein synthesis were measured, and results showed that post-exercise ingestion of a combined CHO and protein supplement does enhance protein accretion. Authors speculate that this finding is due largely to the presence of circulating insulin, which increases in blood immediately after CHO ingestion, and is critical in the regulation of protein synthesis (2) and proteolysis (12).
The GI of the CHO consumed later in the post-exercise period, has also be show to be important because of their influence on substrate oxidation. In 2005, Stevenson et al. conducted a study to examine the metabolic responses to high GI or low GI meals consumed during recovery from prolonged exercise. During this study, eight trained male athletes participated in a crossover design composed of two trials separated by at least seven days. In both trials, subsequent to an overnight fast, subjects completed a 90 minute run at 70% VO2max, with meals provided thirty minutes and two hours following cessation of exercise. Following the ingestion of the second meal, serum insulin concentration was higher in the high-GI trial as compared to the low GI trial. Results suggest that a low GI diet may be more beneficial for continued utilization of fat during the later recovery period (22).
In 2004, Burke and colleagues put forth revised guidelines for the intake of CHO in the everyday or training diets of athletes (6). These recommendations focus on the practical application of substrate utilization research. As an extension to this work, we feel that recommendations for the intake of CHO in terms of GL would prove a useful tool for the athlete or professionals working with athletes. We have begun this process by merging one of the most iconic representations of nutrition education with such recommendations, with the end product being The Glycemic Load Food Guide Pyramid for Athletes (Figure 1). This tool is designed to be used in the lay literature to help athletes and professionals working with athletes understand how to apply the concept of GL to optimal athletic performance.
Because numerous methodological problems exist in the definition of GI and GL, including poor standardization, poor reproducibility, and high variability, (1) it was paramount to use a single, reliable measure of GI/GL on which to base our recommendations. In 2002, Foster-Powell et al. published an extensive table of GI and GL values for over 750 different foods (13). This table compiles all relevant data published between 1981 and 2001, and for multiple listings of a food, means and standard deviations have been calculated. It is this table upon which our classification of GL is based.
Future research addressing the role of GL as a strategy for exercise enhancement needs to focus of how it may enhance athletic performance. Too often we assess improvements in athletic performance with enhanced strength or endurance capacity, and this may not translate into enhancement of performance. While such studies prove beneficial, studies designed to capture athletic performance outcomes are needed.
Additionally, more research needs to be conducted looking at how GI and GL values change when food is consumed in a non-fasted state. Recall that the GI is calculated by measuring the incremental area under the blood glucose curve following ingestion of a test food providing 50 g of CHO, compared with the area under the curve following an equal CHO intake from the reference food, with all tests being conducted after an overnight fast. Seldom in practice will athletes fast prior to a competition, let alone during and after. How GI may change in the non-fasted state is of critical importance to the athlete.
Finally, research and development in the food science arena needs to focus on palatable low glycemic foods specifically engineered for the athlete. Previous studies assessing the effect of low glycemic foods on athletic performance have historically chosen lentils - often consumed in very large portions - which may not be a practical choice for the athlete for myriad reasons. Although low GI/GL precompetition meals have been shown to prolong endurance during strenuous exercise, if accompanied by gastric distress, the athlete is unlikely to follow such recommendations.▪
1. Arteaga Llona, A. The glycemic index
. A current controversy. Nutr Hosp
21 Suppl 2: 53-59, 55-60, 2006.
2. Biolo, G, Declan Fleming, RY, and Wolfe, RR. Physiologic hyperinsulinemia stimulates protein synthesis and enhances transport of selected amino acids in human skeletal muscle. J Clin Invest
95: 811-819, 1995.
3. Brand-Miller, J, Hayne, S, Petocz, P, and Colagiuri, S. Low-glycemic index
diets in the management of diabetes: a meta-analysis of randomized controlled trials. Diabetes Care
26: 2261-2267, 2003.
4. Burke, LM, Claassen, A, Hawley, JA, and Noakes, TD. Carbohydrate intake during prolonged cycling minimizes effect of glycemic index
of pre-exercise meal. J Appl Physiol
85: 2220-2226, 1998.
5. Burke, LM, Collier, GR, and Hargreaves, M. Glycemic index
-a new tool in sport nutrition
? Int J Sport Nutr
8: 401-415, 1998.
6. Burke, LM, Kiens, B, and Ivy, JL. Carbohydrates and fat for training and recovery. J Sports Sci
22: 15-30, 2004.
7. Chryssanthopoulos, C, Williams, C, Nowitz, A, Kotsiopoulou, C, and Vleck, V. The effect of a high carbohydrate meal on endurance running capacity. Int J Sport Nutr Exerc Metab
12: 157-171, 2002.
8. Ciok, J and Dolna, A. The role of glycemic index
concept in carbohydrate metabolism. Przegl Lek
63: 287-291, 2006.
9. Costill, DL, Sherman, WM, Fink, WJ, Maresh, C, Witten, M, and Miller, JM. The role of dietary carbohydrates in muscle glycogen resynthesis after strenuous running. Am J Clin Nutr
10. Coyle, EF. Substrate utilization during exercise in active people. Am J Clin Nutr
61(4 Suppl): 968S-979S, 1995.
11. Coyle, EF, Coggan, AR, Hemmert, MK, and Ivy, JL. Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate. J Appl Physiol
61: 165-172, 1986.
12. Flakoll, PJ, Kulaylat, M, Frexes-Steed, M, Hourani, H, Brown, LL, Hill, JO, and Abumrad, NN. Amino acids augment insulin's suppression of whole body proteolysis. Am J Physiol
257(6 Pt 1): E839-47, 1989.
13. Foster-Powell, K, Holt, SH, and Brand-Miller, JC. International table of glycemic index
and glycemic load
values: 2002. Am J Clin Nutr
76: 5-56, 2002.
14. Ivy, JL, Lee, MC, Brozinick Jr, JT, and Reed MJ. Muscle glycogen storage after different amounts of carbohydrate ingestion. J Appl Physiol
15. Jenkins, DJ, Wolever, TM, Taylor, RH, Barker, H, Fielden, H, Baldwin, JM, Bowling, AC, Newman, HC, Jenkins, AL, and Goff, DV. Glycemic index
of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr
34: 362-366, 1981.
16. Jentjens, R and Jeukendrup, A. Determinants of post-exercise glycogen synthesis during short-term recovery. Sports Med
33: 117-144, 2003.
17. Jimenez-Cruz, A, Gutierrez-Gonzalez, AN, and Bacardi-Gascon, M. Low glycemic index
lunch on satiety in overweight and obese people with type 2 diabetes. Nutr Hosp
20: 348-350, 2005.
18. Levenhagen, DK, Gresham, JD, Carlson, MG, Maron, DJ, Borel, MJ, and Flakoll, PJ. Postexercise nutrient intake timing in humans is critical to recovery of leg glucose and protein homeostasis. Am J Physiol Endocrinol Metab
280: E982-993, 2001.
19. Schabort, EJ, Bosch, AN, Weltan, SM, and Noakes, TD. The effect of a preexercise meal on time to fatigue during prolonged cycling exercise. Med Sci Sports Exerc
31: 464-471, 1999.
20. Sherman, WM, Brodowicz, G, Wright, DA, Allen, WK, Simonsen, J, and Dernbach, A. Effects of 4 h preexercise carbohydrate feedings on cycling performance. Med Sci Sports Exerc
21: 598-604, 1989.
21. Sloth, B, Krog-Mikkelsen, I, Flint, A, Tetens, I, Björck, I, Vinoy, S, Elmståhl, H, Astrup, A, Lang, V, and Raben, A. No difference in body weight decrease between a low-glycemic-index and a high-glycemic-index diet but reduced LDL cholesterol after 10-wk ad libitum intake of the low-glycemic-index diet. Am J Clin Nutr
80: 337-347, 2004.
22. Stevenson, E, Williams, C, and Biscoe, H. The metabolic responses to high carbohydrate meals with different glycemic indices consumed during recovery from prolonged strenuous exercise. Int J Sport Nutr Exerc Metab
15: 291-307, 2005.
23. Suzuki, M. Glycemic carbohydrates consumed with amino acids or protein right after exercise enhance muscle formation. Nutr Rev
61(5 Pt 2): S88-94, 2003.
24. Thomas, DE, Brotherhood, JR, and Brand, JC. Carbohydrate feeding before exercise: effect of glycemic index
. Int J Sports Med
12: 180-186, 1991.
25. Thomas, DE, Brotherhood, JR, and Miller, JB. Plasma glucose levels after prolonged strenuous exercise correlate inversely with glycemic response to food consumed before exercise. Int J Sport Nutr
4: 361-373, 1994.
26. Thorburn, AW, Brand, JC, and Truswell, AS. The glycaemic index of foods. Med J Aust
144: 580-582, 1986.
27. Wright, DA, Sherman, WM, and Dernbach, AR. Carbohydrate feedings before, during, or in combination improve cycling endurance performance. J Appl Physiol
71: 1082-1088, 1991.